A hole in Earth's protective ozone layer above the Antarctic has become an annual event for the last 25 years, greatly increasing the South Pole's exposure to ultraviolet rays. Now, the same thing is happening above the Arctic Circle.

The ozone layer is found roughly between 20 and 25 miles above our planet's surface. Primarily composed of the oxygen compound ozone, this layer serves to absorb about 97-99% of all ultraviolet light that reaches Earth from the Sun, shielding us from its more dangerous effects. But due to the heavy use of various human-manufactured compounds — in particular chlorofluorocarbons, or CFCs, which have since been largely banned — the ozone layer has become depleted, opening up holes over as much as 5% of the Earth's surface ever since the mid-1980s.

Until now, the vast majority of those holes were found high above Antarctica, but now it appears this severe ozone depletion has migrated to the other pole. An international team of researchers discovered that a mix of ozone-depleting pollutants and unusually cold temperatures above the North Pole drove the creation of a new Arctic ozone hole unlike any we've seen before.

The polar regions tend to attract these holes because of a phenomenon known as the polar vortex, a circulation pattern in the atmosphere caused by cold temperatures and the rotation of the Earth that greatly increases the amount of regular chemicals being converted into ozone-depleting ones. This past year saw particularly cold temperatures and, as a result, an unusually long and severe polar vortex. The result was the creation of way more ozone-depleting chemicals than normal, making the formation of a full-blown ozone hole in the Arctic all but inevitable.

University of Toronto researcher Kaley Walker explains:

"In the 2010-11 Arctic winter, we did not have temperatures that were lower than in the previous cold Arctic winters. What was different about this year was that the temperatures were low enough to generate ozone-depleting forms of chlorine for a much longer period of time. Arctic ozone loss events such as those observed this year could become more frequent if winter Arctic stratospheric temperatures decrease in future as Earth's climate changes."

If you're trying to look on the bright side, the good news is that the Arctic polar vortex is much smaller than its Antarctic counterpart, which means the Northern ozone hole also covers far less ground than the Southern one. But that's about it for good news — the Arctic ozone hole is much more mobile, and it's capable of reaching human population centers in the Arctic Circle that don't exist down in Antarctica. That means the threat posed by the Arctic ozone hole is potentially greater, in the short term, than its Southern counterpart.

The researchers also point out that without the 1989 Montreal Protocol, which banned many of the most severe ozone-depleting pollutants, we would probably have long since reached the point where an Arctic ozone hole forms ever year. As it stands, it's not clear whether this is going to be a regular thing, or represents a relatively unusual occurrence. Either way, it unfortunately isn't likely the last time we'll see such a hole — the effects of CFCs and other chemicals is long-lived and will likely linger on for decades more.